[PROJ] Vertical deflection: of interest for PROJ ?

[Joel Haasdyk]

Great question Even,
Prompted me to review for myself...

As you point out yourself, a 50" deflection of the vertical would result in a deviation of 0.24m at a dHeight of 1000m.
However even at dHeight of only 10m, that means 2.4mm.
As Nyall points out... one man's noise is another man's signal; A couple of mm is a long way, in some applications.
I too would suggest/request that PROJ makes use of the best available information where possible.


Will E. Featherstone & Jean M. Rüeger (2000) The importance of using deviations of the vertical for the reduction of survey data to a geocentric datum, Australian Surveyor, 45:2, 46-61, DOI: 10.1080/00050326.2000.10440341 (available from https://www.tandfonline.com/doi/abs/10.1080/00050326.2000.10440341 or by request to the author, perhaps), 
describes these issues (and more) in detail. That paper aims to remind surveyors that modernising to a global ellipsoid (GRS80) would require use of these corrections, where they were previously largely ignored when using a local Australian ellipsoid (ANS).
It appears that you have likely read this document already;  the 50" deviation in your example is also the maximum deflection expected across the Australian continent (with average 6.8"), after it switched from a local to a global ellipsoid.

The paper discusses the importance for the SURVEY community in applications including:
     1. transformation between astronomical coordinates and geodetic coordinates;
     2. conversion between astronomic or gyro azimuths and geodetic azimuths;
     3. reduction of measured horizontal directions (and angles) to the ellipsoid;
     4. reduction of measured zenith angles to the ellipsoid;
     5. reduction of slope electronic distance measurements (EDM) to the ellipsoid using zenith angles; and
	[See table 10]
     6. determination of height differences from zenith angles and slope distances.
	[See table 16]

Some of the above (e.g. #1, #5, #6) are directly related to your coordinate transformation question.
[Apologies that I can't attach to link the source doc. It is for purchase, so it is not for me to distribute!]

Joel Haasdyk | GDA2020 Implementation Program Manager (NSW)
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-----Original Message-----
From: PROJ <proj-bounces at lists.osgeo.org> On Behalf Of Nyall Dawson
Sent: Thursday, 9 January 2020 9:37 AM
To: Even Rouault <even.rouault at spatialys.com>
Cc: PROJ <proj at lists.osgeo.org>
Subject: Re: [PROJ] Vertical deflection: of interest for PROJ ?

On Thu, 9 Jan 2020 at 06:38, Even Rouault <even.rouault at spatialys.com> wrote:
>
> Hi,
>
> While integrating different datasets over the last months, I've 
> noticed that a few geoids (like the USA or Australian geoids) are 
> accompanied with the angles for the vertical deflection. I've read a 
> bit about that, but I'm still unclear if such data would be useful for 
> what PROJ does, that is coordinate transformations.
>
> For the formula converting between ellipsoidal height to orthometric 
> height, H = h - N, sometimes instead of the = one sees a ~= . My 
> understanding of the typical deflection angles is that the 
> approximation is legitimate, as being largely a sub millimetric 
> phenomenon, given that the cosinus of small angles is super close to 1.
> But would there be an impact on horizontal coordinates ?
> For example for a difference of height of 1000 m and a deflection of
> 50 arc second, that would be 1000 * tan(50'') = 0.24 m But perhaps the 
> use case for a PROJ-like context could be "given a skyscrapper
> 1 km high whose facade is vertical along the plumb line, given the 
> coordinates at the bottom, what are the coordinates at the top ?", 
> which doesn't seem to be something people would use routinely :-)

Hmmm... you say that now, but with the ever blurring line between CAD/GIS/BIM I think it IS likely to be a requirement in the near future!

Nyall
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-----Original Message-----
From: PROJ <proj-bounces at lists.osgeo.org> On Behalf Of Even Rouault
Sent: Thursday, 9 January 2020 7:39 AM
To: proj at lists.osgeo.org
Subject: [PROJ] Vertical deflection: of interest for PROJ ?

Hi,

While integrating different datasets over the last months, I've noticed that a few geoids (like the USA or Australian geoids) are accompanied with the angles for the vertical deflection. I've read a bit about that, but I'm still unclear if such data would be useful for what PROJ does, that is coordinate transformations.

For the formula converting between ellipsoidal height to orthometric height, H = h - N, sometimes instead of the = one sees a ~= . My understanding of the typical deflection angles is that the approximation is legitimate, as being largely a sub millimetric phenomenon, given that the cosinus of small angles is super close to 1.
But would there be an impact on horizontal coordinates ?
For example for a difference of height of 1000 m and a deflection of 50 arc second, that would be 1000 * tan(50'') = 0.24 m But perhaps the use case for a PROJ-like context could be "given a skyscrapper
1 km high whose facade is vertical along the plumb line, given the coordinates at the bottom, what are the coordinates at the top ?", which doesn't seem to be something people would use routinely :-)

Even

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